Pervaporation is a separation process where a membrane in contact with a multi-component liquid feed selectively absorbs one or more of the species from the feed. The sorbed species permeate across the membrane under the influence of a concentration gradient that is produced by evaporating the sorbed molecules from the product side of the membrane using a vacuum or sweep gas. Permeate vapor is then condensed and recovered as a liquid. Vapor permeation differs from pervaporation in that the feed is already in the vapor phase.
U.S. Pat. No. 5,753,008, incorporated by reference herein, discloses three ways for maintaining a sufficiently low partial pressure of permeate on the downstream side (also referred to as the “permeate side”) of a membrane in a vapor permeation process: (a) by vacuum, (b) by dilution, and (c) by countercurrent sweep. When the low partial pressure is obtained by a vacuum or partial vacuum, it is conventional (i.e., known in the art of separations with polymer membranes) to use vacuum pumps and/or steam ejectors (including multi-stage steam ejectors).
While effective, vacuum pumps are expensive to maintain and can lead to difficulty in condensing the permeate. For this reason, pervaporation systems have been disclosed which use refrigeration equipment to assist in permeate condensation. Moreover, the vacuum pump oil may become contaminated with permeate, thus requiring more maintenance. And as a result of their generally low displacement, vacuum pumps are not typically useful in large separations systems.
While simpler, steam ejectors are limited in the level of vacuum they can achieve at reasonable steam flow rates. Consequently, staged ejectors are often needed for pervaporation separation systems. And since a small amount of permeate can remain in the steam condensate effluent, a treatment of the water effluent may be needed before discharge.
U.S. Pat. No. 6,273,937 attempts to overcome these difficulties by using a Venturi-type nozzle to produce the vacuum on the downstream side of the membrane. The Venturi is operated using a non-volatile working fluid that has an affinity for the permeate molecules. Since the working fluid has an affinity for the permeate molecules, the product effluent stream is heated and a flash drum is used to separate the permeate molecules, which can then be cooled and conducted away from the process. Following permeate separation in the flash drum, the non-volatile working fluid is cooled and pumped to the inlet of the Venturi for re-use. While the disclosed process achieves a vacuum on the downstream side of the membrane, it is complicated by the method selected to separate permeate from the Venturi effluent, as is also the case of the steam ejectors of U.S. Pat. No. 5,753,008.
There is, therefore, a need for a membrane separation process having an improved method for providing a vacuum on the downstream side of the membrane.